The present invention is directed to a new and improved method and associated device for calibrating sound intensity probes. A plurality of sound intensity probes when so calibrated can be adapted for use as discrete measurement points comprising a portable probe array designed to accomplish rapid in situ sound testing of a test object in an environment of ambient background noise.
Calibration is imperative to the usefulness of a mutually spaced pair of condenser type microphones adapted for use as a sound intensity probe. Calibration is even more critical as there are no present standards for sound intensity measurements. Particularly lacking are standards for in situ measurements. Conventional sound intensity probes are typically phase matched by the manufacturer--a careful and delicate physically altering process which escalates the cost of these commercially available probes into the $5000 to $10,000 range.
The technique of calculating sound power flow from a sound source using spectral analysis is recognized as an in situ method for measuring sound emission of products in a production line setting. In situ sound testing involves taking measurements on a test object where it lies--amidst ambient background noise. For in situ sound power testing applications, the background environment is not merely undesirable signal distortion to be disregarded, but part of the measurable signal, to be retained and eventually averaged away through calculation of the total emitted sound power. The technique requires that time histories for each microphone pair of a plurality of sound intensity probes be made over the same time interval and collected over a sufficiently distributed array of such probes.
Heretofore, such a measurement collection scheme was in practice prohibited by the excessive cost of securing a plurality of commercially available phase matched sound intensity probes. The calibrator and calibration method disclosed herein offer a way of calibrating common, inexpensive, unmatched microphone pairs for utilization as pressure transducing sound intensity probes. Each probe's gain and phase calibration factors are independently obtained and stored as part of an external data base for subsequent utilization. Calibration correction can be linearly applied in the direct signal processing required to determine sound intensity at each probe. The calibration data base as obtained for a plurality of probes which have been adapted for use in an arbitrary test measurement array, is used to computationally compensate for phase distortion in time histories simultaneously collected from each probe microphone. The use of independent calibration factors permits simultaneous correlation of the entire plurality of array probes in a practical manner. Furthermore, the technique produces results comparable to those heretofore only obtainable from expensive, high quality, commercially available sound intensity probes. The reader is referred to the above mentioned application Ser. No. 07/612,937 for further discussion of this compensation technique as applied to in situ sound testing.
Generally, there are two separable components contributing to phase distortion between electrical signals transduced from the displacement of respective diaphragms of a condenser microphone pair comprising a typical sound intensity probe. One is a sound field component due to the passage of acoustic energy waves through the sound field itself. The other is distortion due to physical transduction of the signal. Transduction distortion introduced by physical differences between the microphones of the probe itself are constant and time invariant. Furthermore, this type of phase distortion is independent and separable from sound field distortion. The invariance coupled with separability allows a calibration correction to be implemented. Calibration is distinguished from performance monitoring; in that only invariant, linearly separable errors can be calibrated. Once determined, calibration correction factors can be independently stored for subsequent application during signal processing.
The calibrator disclosed herein controls the sound field component of phase distortion so that the physical transduction component can be more accurately quantified by a calibration factor independent of any other systematic distortion. This independent calibration technique provides a simple linear computational means to compensate for phase mismatch between microphone pairs in any number of probes.
It is therefore an object of the present invention to better control temporal sound field distortion through improved calibrator design in order to more accurately quantify calibration of phase mismatch due to physical differences between the pair of microphones comprising the probe.
It is another object of the invention that the calibrator and calibration technique be rugged, portable, easy to use and rapid to accommodate frequent on site calibration checks of probes.
It is further an object of the invention that the calibration technique be adapted to simultaneously calibrate any number of discrete measurement points comprised of a plurality of probes for in situ testing using independently derived calibrator correction factors.
It is yet another further object of the invention that the calibration scheme accommodate the on site need to change or replace probe microphone(s) in the field with the capability of rapid and reliable recalibration. Such capability allows the response character of the probe to be altered by replacing either or both microphones and/or the associated mutual spacer.